1. Field of the Invention
The present invention relates to a steel cord for radial tires, and more particularly relates to a steel cord for radial tires having a construction including a "2+5" bi-diametric cord formed by twisting a core and a strand in such a fashion that they have the same twist pitch length and the same twist direction, so that the number of processing steps for the twisting process is reduced from two to one, thereby reducing the manufacturing costs while achieving an improvement in productivity.
2. Description of the Prior Art
In a steel cord having a two-layer twisted construction, a core, which forms a first layer of the steel cord, and a strand, which forms a second layer of the steel cord, are typically different from each other in terms of twist pitch length and twist direction as follows:
Steel Cord Construction: 1.times.3(0.20)+6(0.35) PA1 Twist Pitch Length of Core: 10 mm PA1 Twist Pitch Length of Strand: 18 mm PA1 Twist Direction of Core: S (Right) PA1 Twist Direction of Strand: Z (Left) PA1 Steel Cord Construction: 1.times.2+7(0.30) PA1 Twist Pitch Length of Core: 8 mm PA1 Twist Pitch Length of Strand: 16 mm PA1 Twist Direction of Core: S (Right) PA1 Twist Direction of Strand: S (Right)
For this reason, it is impossible to carry out the twisting process for the manufacture of such a cord using a single processing step. Conventionally, the twisting process has been carried out using two processing steps. However, this results in an increase in the manufacturing costs for cords. Furthermore, the contact between the core and strand is rendered to be a point contact. Where such a cord is applied to a tire, a stress concentration occurs at portions of the cord, where point contacts are formed, during the use of the tire. Due to such a stress concentration, a fretting phenomenon occurs which results in a degradation in the durability of the tire.
In order to reduce the above mentioned drawback, a method for the manufacture of steel cords having a two-layer twisted construction has been proposed in which a steel cord is formed by twisting a core and a strand in such a fashion that they have different twist pitch lengths while having the same twist direction as follows:
In this case, it is still impossible to carry out the twisting process for the manufacture of such a cord using a single processing step because the core and strand are different in terms of twist pitch length. Consequently, the twisting process has been carried out using two processing steps in accordance with this method. For this reason, an increase in the manufacturing costs for cords is still involved. Although line contacts are formed between the core and strand by virtue of the same twist direction, they are unstable because the core and strand have different twist pitch lengths. In other words, although this method can reduce the durability degradation of the tire to a some degree, it has a limitation in completely eliminating the fundamental problems.
Meanwhile, in the case of a cord layered with topping rubber, an insufficient penetration of the topping rubber into the core layer may occur. Where such a cord is applied to a tire, such an insufficient penetration of the topping rubber results in a movement of the core layer. As a result, corrosion of the cord caused by moisture or salt is accelerated, thereby greatly degrading the durability of the tire.